The present invention relates to a priority type flow dividing valve, which is suitable in utilizing a pressurized fluid of a common pressurized hydraulic source, for steering and loading controls in a forklift or the like.
For example, in a forklift or the like, a pressurized fluid from a single hydraulic source is divided by a flow dividing valve to obtain a predetermined amount of the fluid to be used primarily for steering, and at the same time, the rest of the fluid is used for loading. As described above, a valve which operates to obtain or secure a predetermined amount of fluid primarily for a special purpose, such as operating a steering, is called a priority type flow dividing valve.
A conventional priority type flow dividing valve is formed of a housing having a high pressure port, a steering port and a loading port; a spool slidably situated in the housing to lead a pressurized fluid in the high pressure port to the steering port through a control orifice and also to directly lead the fluid to the loading port; a spring capable of providing an elastic force to the spool in a direction opposite to a liquid pressure force which works on the spool in accordance with a differential pressure of the control orifice at the front and rear sides; a first throttle disposed between the spool and housing to open and close a path to the steering port in accordance with advancing and retreating movements of the spool; and a second throttle disposed between the spool and the housing and actuated oppositely relative to the first throttle in accordance with the advancing and retreating movements of the spool so as to open and close a path to the loading port. In the conventional priority type flow dividing valve, a predetermined amount of the pressurized fluid is primarily led to the steering port, and if necessary, a surplus flow or amount of the pressurized fluid is divided to flow to the loading port.
Namely, a priority flow called PF flow amount flows through the control orifice, but at this time, pressure difference occurs at the front and rear sides of the control orifice. The pressure difference allows the spool to move while it compresses a spring set at the predetermined spring characteristic in advance, but while balancing the first and second throttles disposed between the spool and the housing, the spool is finally held at a position such that the differential pressure and a spring force are balanced. The first throttle is a so-called PF throttle for opening and closing the path to the steering port, and the second throttle is a so-called MF throttle for opening and closing the path to the loading port.
The both throttles are switched in connection with high-low degree of the steering operation pressure and the loading operation pressure. When the steering pressure, i.e. pressure at the steering port, is high, the MF throttle functions, and when the loading operation pressure, i.e. pressure for the loading port, is high, the PF throttle functions, to thereby maintain the PF flow rate at the predetermined amount.
On the other hand, the control orifice, the PF throttle, the MF throttle and the spring constitute a feedback control system for controlling the spool, and the spool always has a vibration, called hunting. Especially, in the various kinds of hydraulic pressure control type steering systems, for which the flow dividing valve is used, a full hydraulic pressure type steering system is weak in strength in a vibration system, and is liable to be uncontrollable by oscillation or vibration. Thus, conventionally, a damper chamber, which enlarges or shrinks while the pressurized fluid in the high pressure port flows in or out through a damper orifice, is formed between the spool and the housing, and oscillation is prevented by utilizing the resistance through the damper orifice.
However, if the diameter of the damper orifice is reduced to prevent the oscillation, to raise a resistance against the pressure fluid and to increase a decrement suppressing effect, when switching between the loading condition functioned in the PF throttle and the steering condition functioned in the MF throttle is made, the resistance against the movement of the spool is increased to cause a time delay in the movement of the spool. Then, by the time delay, the switching between the steering condition and the loading condition is delayed, to cause an extremely bad effect to the steering operation, which is considered to be important especially in view of operability and safety.
In case a forklift is referred to as an example described above, when the diameter of the damper orifice is enlarged to move the spool smoothly, the differential pressure between the front and rear of the orifice, i.e. resistance against the pressurized fluid, is reduced, so that the oscillation in the steering system is likely to occur to fall into an inoperable condition. On the other hand, when the diameter of the damper orifice is reduced, at the time of changing to or from the steering operation condition, the movement of the spool is delayed to cause an action by the pressure, so that there might be a problem such that a phenomena, such as an impact or kickback to the handle, occurs.
In order to solve the aforementioned problems, an object of the present invention is to provide a priority type flow dividing valve which can be operated safely in any conditions.
Another object of the invention is to provide a priority type flow dividing valve as stated above, wherein the valve condition can be switch smoothly.
Further objects and advantages of the invention will be apparent from the following description of the invention.